test: stop mobile touch-pan screenshot flaking on text anti-aliasing (#15356)

## Problem

`browser_tests/tests/vueNodes/interactions/canvas/pan.spec.ts:204` —
`@mobile Can pan with touch`, run by the `mobile-chrome` Playwright
project — fails intermittently across unrelated PRs with:

```
40 pixels (ratio 0.01 of all image pixels) are different.
```

Across 9 open PRs the `playwright-tests (mobile-chrome)` check failed on
3 and passed on 6. On one PR it failed, was re-run with no code change,
and passed. Two of those failures
([#15352](#15352),
[#15339](#15339)) are
this flake and produced **byte-identical** actual/diff PNGs on different
runners. The third (#15337) is a genuine rendering change from that PR —
it moved 3146 pixels here and also broke 4 `mobileBaseline.spec.ts`
screenshots, so it is not an instance of this flake.

## Root cause

I pulled the `playwright-report-mobile-chrome` artifacts and located the
40 differing pixels in the diff image:

- Bounding box `x ∈ [202, 348]`, `y ∈ [612, 617]` — inside the glyphs of
the Load Checkpoint node's `ckpt_name` combo widget value,
`v1-5-pruned-emaonly-fp16.sa…`.
- **Every other pixel of the 393×727 image is byte-identical** between
the passing and failing renders — the node's `transform: translate(26px,
444px)`, its error ring, the `ckpt_name` label, the node title, the
toolbar and the canvas grid all match exactly.
- Glyph positions are identical (per-glyph ink-centroid drift < 0.09
px). The failing render simply has ~5% less ink — the same glyphs at the
same coordinates rasterized through a different text-antialiasing path.

So this is **not** an unsettled pan, a mid-frame screenshot, or
momentum: the pan transform is fully deterministic. It is also not
something a readiness signal can wait out — Playwright reports `captured
a stable screenshot` and then re-fails all 3 retries with the exact same
40 pixels.

## Fix

Two changes, neither of which touches the baseline:

1. **Assert the pan actually landed** before comparing pixels, matching
the sibling tests in this file. A pan that never applies now fails with
a readable offset assertion instead of surfacing as a pixel diff.
2. **Bound the comparison with `maxDiffPixels: 100`**, in line with
existing usage in `selectionToolbox.spec.ts`, `interaction.spec.ts`,
`widget.spec.ts` and `canvasSettings.spec.ts`.

### Why not the alternatives

- **Wait on a ready signal** — nothing is unsettled. The geometry is
byte-identical and Playwright already burns 5s of retries on a
screenshot it considers stable.
- **Disable animations** — already the default (`animations:
'disabled'`).
- **Mask the region** — would blank a genuine part of the assertion and
require regenerating the baseline.
- **Product bug in the pan transform** — ruled out; the transform is
identical in both renders.

## Before / after

Replaying the real CI bytes through Playwright's own image comparator
(`playwright-core` `getComparator('image/png')`) against the committed
baseline:

| Input | Before | After |
| --- | --- | --- |
| CI failing render (#15352 / #15339, byte-identical) | **FAIL** — 40 px
| **PASS** |
| CI passing render | PASS | PASS |
| Control: #15337 real rendering change | FAIL — 3146 px | **FAIL** —
3146 px |
| Control: 1px pan error (dx=1) | FAIL — 3615 px | **FAIL** — 3615 px |
| Control: 1px pan error (dy=1) | FAIL — 5845 px | **FAIL** — 5845 px |

The tolerance flips exactly the flake and nothing else. The smallest
real regression measurable on this test is a 1-pixel pan error at 3615
px — 36× the bound and 90× the observed noise.

### Verification caveat

I did not run `--repeat-each` locally. The baseline is
`mobile-chrome-linux`, generated in the CI container, so a local run
would mismatch on unrelated font rendering and would give no signal on
this specific 40-pixel difference. The table above is a stronger check:
it is a deterministic replay of the exact pixels that turned CI red,
through the same comparator Playwright uses. Confirmation that CI is
green comes from this PR's own `mobile-chrome` run.

Author: christian-byrne

browser_tests/tests/vueNodes/interactions/canvas/pan.spec.ts

@@ -205,12 +205,22 @@ test.describe('Vue Nodes Canvas Pan', { tag: '@vue-nodes' }, () => {
     '@mobile Can pan with touch',
     { tag: '@screenshot' },
     async ({ comfyPage }) => {
+      const offsetBefore = await comfyPage.canvasOps.getOffset()
+
       await comfyPage.canvasOps.panWithTouch(
         { x: 64, y: 64 },
         { x: 256, y: 256 }
       )
+
+      // Fail on a pan that never landed here, not as a screenshot diff.
+      await expect
+        .poll(() => comfyPage.canvasOps.getOffset())
+        .not.toEqual(offsetBefore)
+
+      // Tolerates text anti-aliasing noise in the widget value text.
       await expect(comfyPage.canvas).toHaveScreenshot(
-        'vue-nodes-paned-with-touch.png'
+        'vue-nodes-paned-with-touch.png',
+        { maxDiffPixels: 100 }
       )
     }
   )